The goal is to study the abnormal differentiation of leukocytes and to establish better methods for the diagnosis and classification of leukemias and lymphomas. Recently, using a modification of our plastic embedding technique, we analyzed the distribution of nine enzymes (5'-NT, ATP, alkaline and acid phosphatase, beta-glucuronidase, alpha-NAE, alpha-NBE, CAE, and peroxidase) in human lymphoid tissue and clearly showed the ability of this technique to accurately identify macrophages in these tissues. Furthermore, several stromal cells can be labeled (e.g., the fibroblastic reticulum cells of the paracortex with alkaline phosphatase and the dendritic reticulum cell with 5'NT). Using this technique, anaplastic seminomas and large cell lymphomas may be easily distinguished in that the cells of the germ cell malignancy are outlined with alkaline phosphatase. We are currently expanding the number of distinguishing cell markers on plastic embedded pathologic biopsy material by applying immunocytochemical methods. We have developed a well-defined phenotype for definition of Histiocytosis X(ANAE+, AcP+, AcP+, Ia+, OKT6+) and demonstrated its identity with human Langerhans cells. Using a similar approach we have shown the Kaposi's sarcoma is mainly an unusual proliferation of lymphatic endothelium. (4)